1. Field of the Invention
This patent specification relates to a method and apparatus for image forming, and more particularly to a method and apparatus for image forming capable of effectively performing a color displacement detection.
2. Discussion of the Background
Conventionally, color image forming apparatuses that form a color image using a number of different color toners often cause a defective phenomenon that images of different color toners have displacements relative to each other. This typically causes a blurred color image. Therefore, these color image forming apparatuses are required to adjust positions of color images to precisely form a single color image with an appropriate color reproduction.
Japanese Patent No. 2573855, for example, describes an exemplary color position adjustment and a test pattern used in the color position adjustment. Also, several other test patterns are described in published Japanese unexamined patent applications No. 11-65208, No. 11-102098, No. 11-249380, and No. 2000-112205. In the image forming apparatuses disclosed in these documents, a plurality of photosensitive drums form a predetermined test image pattern using a plurality of color toners on both longitudinal sides of an image carrying surface of an image carrying member. The predetermined test pattern is detected by a pair of optical sensors. Based on this detection, displacements of color images relative to each other are calculated and are used to justify the positions of the color image layers. More specifically, the predetermined test pattern includes a plurality of marks and the reading of the marks allows an analysis of a displacement of each color from a predetermined reference position. For example, the color position adjustment calculates a displacement dy in a sub-scanning direction y, a displacement dx in a main scanning direction x, a displacement dLx of an effective line length in a main scanning line, and a skew dSq in the main scanning line.
In the above-mentioned optical sensor, a photo-electronic conversion element such as a phototransistor, for example, receives the light reflected from or passing through the transfer belt via a slit and converts it into a voltage as an analog detection signal indicating an amount of the received light. The detection signal is corrected with an operational amplifier, for example, and has a predetermined voltage range. For example, the detection signal is varied to high (H) for 5 volts, for example, when no mark is detected and is varied to low (L) for 0 volts, for example, when a mark is detected. The varying curve of the detection signal depends on a relationship among a moving speed of the transfer belt, a width of the mark formed on the transfer belt, and a width of the slit. When the transfer belt is moved at a constant moving speed and the width of the mark is sufficiently greater than the width of the slit, the detection signal will be output in an ideal manner as illustrated in FIG. 21. That is, the detection signal gradually rises from L to H when a leading edge of the mark comes inside a view field of the sensor via the slit, and stays at H while the mark entirely covers the slit. The detection signal then starts to drop to L when a trailing edge of the mark comes out of the view field of the sensor and stays at L until the next mark comes closer.
By providing a threshold value such as 2.5 volts, for example, to the detection signal of FIG. 21, the detection signal can be converted into time-sequential binary data representing a number of pairs of descending and ascending signal parts of a mark. Therefore, a pattern of the marks detected by the optical sensors can be read by analyzing the above-mentioned time-sequential binary data.
However, the above-mentioned method has a drawback that the detection signal cannot be stably output since the frequency of the signal is relatively high and the amplitude is relatively large. In addition, the level of the detection signal often differs from one color to another. Although high frequency noises can be suppressed with a low-pass filter, an excessive suppression results in large variations of the width of the signal while the signal is L and it becomes difficult to recognize a pattern of the marks. Such a problem is prone to be made greater as the transfer belt becomes dusty and wears out. Accordingly, the mark pattern detection for the color displacement adjustment becomes unusable at a relatively early time even if the transfer belt has a relatively long lifetime.
There is an attempt to obtain a mark pattern by repeatedly converting the detection signal with an A/D converter in a relatively short time period, storing the data in a memory, and specifying the positions of data groups distributed in a form corresponding to a reference waveform through a frequency analysis of the detection signal based on the data stored in the memory or through a matching check relative to the reference waveform. However, this attempt has a drawback that an amount of data to be obtained is great and a relatively large memory capacity is required. In addition, a process for specifying the pattern is complex and takes a relatively long time period.
In general, the positions of the marks on the transfer belt are prone to be varied in the moving direction of the transfer belt. For example, when the photosensitive drum or a driving roller of the transfer belt has a shaft deformed, the mark positions are shifted accordingly. In particular, the above-mentioned published Japanese unexamined patent applications No. 11-65208 and No. 11-249380 describe proposals to reduce an error in detecting the color displacement caused by the above-mentioned problem. However, even with the methods described in these proposals, the color displacement detection takes a relatively long time and the reliability of the resultant mark displacement measurements is relatively low.
This patent specification describes a novel method of color displacement detection for use in a color image forming apparatus that sequentially forms image layers in different colors on a photosensitive member and overlays the image layers on one after another into a color image on a transferring member. In one example, this novel method includes the steps of forming, detecting, and calculating. The forming step forms a predetermined number of mark sets within one circumferential length surface of the transferring member. Each of the predetermined number of mark sets includes a predetermined number of different color marks arranged in a line in a moving direction of the transferring member. The detecting step detects the predetermined number of mark sets formed on the transferring member. The calculating step calculates mean values of displacement amounts of the same color marks in different mark sets in the predetermined number of mark sets relative to respectively corresponding reference positions.
The forming step may form the same color marks in the different mark sets included in the predetermined number of mark sets in a pitch of three fourth circumferential length of the photosensitive member.
The predetermined number of mark sets may be eight or four.
The predetermined number of different color marks may be four and the different colors may include magenta, cyan, yellow, and black.
The above-mentioned novel method may further include the steps of converting, storing, and generating. The converting step converts a mark signal output from the detecting step into mark edge data with an A/D conversion using a predetermined pitch. The storing step stores the mark edge data in association with respective scanning positions into a memory. The generating step generates information of mark distribution based on data groups of the mark edge data belonging to two adjacent scanning positions and to signal areas having levels with predetermined varying trends.
This patent specification further describes a novel color displacement detecting apparatus for use in a color image forming apparatus that sequentially forms image layers in different colors on a photosensitive member and overlays the image layers on one after another into a color image on a transferring member. In one example, this novel apparatus includes a pattern generator, a detector, an A/D converter, and a controller. The pattern generator is arranged and configured to generate a test pattern including a predetermined number of mark sets within one circumferential length surface of the transferring member. Each of the predetermined number of mark sets includes a predetermined number of different color marks arranged in a line in a moving direction of the transferring member. The detector is arranged and configured to detect marks included in the predetermined number of mark sets. The A/D converter converts a signal output from the detector into detection data. The controller is arranged and configured to control a storage operation for storing the detection data converted by the A/D converter in association with respectively corresponding scanning positions. The controller is further arranged and configured to calculate positions of marks of the predetermined number of mark sets based on the detection data stored through the storage operation, and to calculate mean values of displacement amounts of the same color marks in different mark sets in the predetermined number of mark sets relative to respectively corresponding reference positions.
In the above-mentioned apparatus, the same color marks in the different mark sets included in the predetermined number of mark sets may be formed in a pitch of three fourth circumferential length of the photosensitive member.
The predetermined number of mark sets may be eight or four.
The predetermined number of different color marks may be four and the different colors may include magenta, cyan, yellow, and black.
This patent specification further describes another novel method of color displacement detection for use in a color image forming apparatus that sequentially forms image layers in different colors on a photosensitive member and overlays the image layers on one after another into a color image on a transferring member. In one example, this novel method includes the steps of generating, detecting, converting, controlling, calculating, and performing. The generating step generates a test pattern including a predetermined number of mark sets within one circumferential length surface of the transferring member. Each of the predetermined number of mark sets includes a predetermined number of different color marks arranged in a line in a moving direction of the transferring member. The detecting step detects marks included in the predetermined number of mark sets. The converting step converts a signal output from the detecting step into detection data. The controlling step controls a storage operation for storing the detection data converted by the converting step in association with respectively corresponding scanning positions. The calculating step calculates positions of marks of the predetermined number of mark sets based on the detection data stored through the storage operation. The performing step performs a calculation of average values of displacement amounts of the same color marks in different mark sets in the predetermined number of mark sets relative to respectively corresponding reference positions.
This patent specification further describes another novel image forming apparatus that sequentially forms image layers in different colors on a photosensitive member and overlays the image layers on one after another into a color image on a transferring member. In one example, this novel apparatus includes an optical writing mechanism and a color displacement detecting mechanism. The optical writing mechanism is arranged and configured to write an image in accordance with image data on the transferring member. The color displacement detecting mechanism includes a pattern generator, a detector, an A/D converter, and a controller. The pattern generator is arranged and configured to generate a test pattern including a predetermined number of mark sets within one circumferential length surface of the transferring member. Each of the predetermined number of mark sets includes a predetermined number of different color marks arranged in a line in a moving direction of the transferring member. The detector is arranged and configured to detect marks included in the predetermined number of mark sets. The A/D converter converts a signal output from the detector into detection data. The controller is arranged and configured to control a storage operation for storing the detection data converted by the A/D converter in association with respectively corresponding scanning positions. The controller is further arranged and configured to calculate positions of marks of the predetermined number of mark sets based on the detection data stored through the storage operation, and to calculate mean values of displacement amounts of the same color marks in different mark sets in the predetermined number of mark sets relative to respectively corresponding reference positions.
This patent specification further described a novel method of image forming that sequentially forms image layers in different colors on a photosensitive member and overlays the image layers on one after another into a color image on a transferring member. In one example, this novel method includes the steps of providing and executing. The providing step provides an optical writing mechanism for writing an image in accordance with image data on the transferring member. The executing step executes a color displacement detection. The executing step further includes the steps of generating, detecting, converting, controlling, calculating, and performing. The generating step generates a test pattern including a predetermined number of mark sets within one circumferential length surface of the transferring member. Each of the predetermined number of mark sets includes a predetermined number of different color marks arranged in a line in a moving direction of the transferring member. The detecting step detects marks included in the predetermined number of mark sets. The converting step converts a signal output from the detecting step into detection data. The controlling step controls a storage operation for storing the detection data converted by the converting step in association with respectively corresponding scanning positions. The calculating step calculates positions of marks of the predetermined number of mark sets based on the detection data stored through the storage operation. The performing step performs a calculation of average values of displacement amounts of the same color marks in different mark sets in the predetermined number of mark sets relative to respectively corresponding reference positions.